B-la cells are atheroprotective; an effect dependent on secreted IgM. Yet, the factors that regulate the B-la subset are unknown. Recent data from our group demonstrates that Id3 is essential for B cell-mediated atheroprotection, and that loss of ld3 resulted in reduced B-la cell number and reduced serum levels of E06 (an atheroprotective natural antibody made by Bla cells). In vivo proliferation assays demonstrated that ld3 was essential for B-la, but not B2 cell homeostatic proliferation and cell culture studies suggest that this effect may be mediated by ld3 regulation of cyclin D2 expression. Accordingly, we hypothesize that Id3 in B cells is necessary for B-la cell atheroprotection through promoting B-la cell cycle progression and self renewal resulting in increased levels of atheroprotective natural antibodies such as E06. This hypothesis and the molecular and cellular mechanisms mediating these effects will be studied in aims 1 and 2. Moreover, we will translate these novel findings in mice into testable hypotheses in humans with type 2 diabetes (T2DM). We have recently identified a single nucleotide polymorphism (SNP) in the human ID3 gene at rs 11574 that encodes an Id3 protein with attenuated function. This SNP was present in 46% of participants in the Diabetes Heart Study and was associated with an increase in carotid intima medial thickness (cIMT) in this diabetic population, suggesting a potential link between loss of Id3 function and atherosclerosis in humans. Recent identification of a novel circulating subset of B cells in humans with the unique properties of murine B-la cells and our new preliminary data suggesting an association of the 1D3 SNP at rsl 1574 with levels of circulating IgM to modified lipids in humans with diabetes, support the hypothesis that Id3 polymorphism at rsl 1574 is associated with reduced circulating B-la-like cells and reduced atheroprotective IgM to modified lipids in humans. Through collaborations with projects 1 and 2 and our human core, we are uniquely poised to test these innovative hypotheses to identify molecular mechanisms regulating atheroprotective B cells in mice and humans, potentially leading to identification of novel biomarkers and novel strategies to bolster innate immune protection against atherosclerosis.